This project will explore a novel hypothesis that arose from our unexpected observation that superimposing muscle paralysis upon exogenous mechanical loading of bone greatly augments the osteogenic response to loading. These data directly challenge a central paradigm in the field, which reflects the consensus that, from the tissue to molecular levels, muscle and bone atrophy and hypertrophy in parallel. Our preliminary studies further demonstrated that the location of the paralyzed muscle relative to the loaded bone did not alter this response, confirming that a circulating factor was responsible for mediating the enhanced anabolism. Our subsequent studies have revealed a candidate for coupling muscle atrophy and bone anabolism, microRNA 206 (mir-206). Consistent with the rapidly expanding literature regarding cell-to-cell signaling via exosomal microRNA and the demonstrated ability of mir-206 to suppress histone deacetylase 4 (HDAC4) which, in turn, enhances osteoblast differentiation, we hypothesize that: muscle paralysis transiently upregulates exosomal mir-206 which, via suppression of HDAC4, enhances osteoblast differentiation and function. We will pursue this thesis through four closely related S. Aims, each with a corresponding sub- hypothesis. The in vivo experiments of S. Aims #1 and #2 are designed to demonstrate that cellular signaling induced by muscle paralysis transiently enhances ongoing osteoblast differentiation and function via exosomes within the serum, that the observed anabolic augmentation can be achieved independently of loading induced osteoblast activation, and that the response can be induced in both trabecular and cortical bone. We will use in vitro approaches to confirm that exosomal mir-206 serves to enhance osteoblast differentiation and function by suppressing translation of HDAC4 (S. Aim #3) and we will demonstrate that downstream in vivo targeting of this pathway (via HDAC4 inhibition) is able to replicate enhanced bone anabolism without the necessity of inducing muscle paralysis (S. Aim #4). If the project is successful, we believe these data would fundamentally alter our conceptualization of how muscle and bone interact at the molecular, cellular, and tissue levels and would enable novel targeting of this pathway as an eventual clinical intervention capable of enhancing bone morphology.